1. Field of the Invention
The present invention relates to a coil made of a superconducting oxide material.
2. Description of the Prior Art
Superconductors have a property that the electric resistance is zero at a temperature less than the transition temperature. Utilizing this property, superconducting electromagnets for generating magnetic fields have already been made fit for practical use by the use of metallic superconductors. Since metallic superconductors have sufficient ductility and malleability, high carrier concentration, and large coherent length, superconducting electromagnets which generate great magnetic fields can be formed by the use of such metallic superconductors. However, in the metallic superconductors a critical temperature at which superconductivity starts (referred to as Tc) is extremely low. Therefore, it was necessary to use liquid helium to maintain the metallic superconductors at a temperature less than the critical temperature in order to produce superconductivity. But the liquid helium has drawbacks that it is so expensive and it is unevenly distributed as a natural resource. On the other hand, recent years, superconducting oxide has been discovered which exhibits superconductivity at a liquid nitrogen temperature or higher. Superconducting electromagnets using such a new superconducting oxide can generate a high magnetic field by making use of liquid nitrogen.
However, there are some problems to be solved when superconducting electromagnets are manufactured by the use of the new superconducting oxide. One is how to form the superconducting oxide which is lack of ductility and malleability into a coil. The other is how to make the coil made of the superconducting oxide which has less grain boundaries in order to improve critical current density of the coil. Concerning the first problem, the superconducting oxide is stuffed in a silver tube used as a sheath material and is wiredrawn, whereby the development of a technique for forming superconducting wires is in progress. On the other hand, concerning the second problem, materials having very few grain boundaries and extremely large critical current density are being developed by means of melting method. However, these two ways to solve the problems are contradictory to each other, so that fundamentally any solutions have not been obtained yet. Namely, superconducting oxide wires formed by means of sheath method using silver tubes have low critical current density in general, and the critical current density falls largely by applying magnetic fields to the wires. On the other hand, the superconducting oxide produced by means of the melting method exhibits sufficiently large critical current density in a magnetic field, but how to form the superconducting oxide into a coil by the use of the melting method has not been entirely researched yet.
Further, since superconducting oxide has low carrier density and extremely short coherent length, grains in the superconducting oxide tend to be electrically connected to each other weakly. For this reason, in superconducting oxide critical current density is extremely low. Compared with the superconducting oxide produced by means of the silver sheath method, the superconducting oxide produced by means of the melting method has large coherent length and high critical current density. Even though electromagnets, namely, closed coils are formed using such superconducting oxide in either method, the coils, except for small coils such as coils having one loop, lose some energy to generate some heat at the connection of both ends of the coils. Therefore, the electromagnets consume large electric power and can not generate large magnetic fields. For example, in the case of connecting both ends of the coils by the use of conductors, the conductors lose some energy due to electric resistance of the conductors themselves and thereby some heat is generated. Therefore, the consumption of electric power is large in such coils and the coils can not generate large magnetic fields.
Here a case of producing a multiturn air-core solenoid coil is taken as an example. The radius of the coil is 10 cm, the inside diameter of the coil is 5 cm, and the length of the coil is 10 cm. A lead wire used for the coil has a cross section of a square having a size of 0.2 mm.times.0.2 mm. When rolling simply the lead wire 1.25.times.10.sup.6 times per meter and applying a current of 1A to the lead wire, the density of the current flowing in the lead wire is 2500A/cm.sup.2, and a magnetic field which this solenoid coil generates is about 1.6 tesla in the center of the coil.
However, the length of the lead wire reaches 6.times.10.sup.6 cm and the resistance of the coil is about 1.5.times.10.sup.4 .OMEGA. in the case where the resistivity of the lead wire is 1.times.10.sup.-6 .OMEGA..cm. The demand of the coil reaches 15 kW.
Such a solenoid coil becomes useless as an electromagnet unless the coil is associated with a cooling apparatus to remove a large amount of heat generated from the coil. However, cooling the coil by the cooling apparatus costs a lot.